Process control networks, such as those used in chemical, petroleum or other processes, generally include a centralized process controller communicatively coupled to one or more field devices which may be, for example, valve positioners, switches, sensors (such as temperature, pressure and flow rate sensors), etc. These field devices may perform control functions within the process (such as opening or closing a valve), may take measurements within the process for use in controlling the operation of the process or may perform any other desired function within the process. Process controllers have historically been connected to field devices via one or more analog signal lines or buses which may carry, for example, 4-20 mA (milliamp) signals to and from the field devices. Generally speaking, the process controller receives signals indicative of measurements made by one or more field devices and/or other information pertaining to one or more field devices, uses this information to implement a typically complex control routine and then generates control signals which are sent via the analog signal buses to one or more of the field devices to thereby control the operation of the process.
Recently, there has been a move within the process control industry to implement field-based digital communications within the process control environment. For example, the process control industry has developed a number of standard, open, digital or combined digital and analog communication protocols such as the HART®, PROFIBUS®, WORLDFIP®, Device-Net® and CAN protocols. These digital communication protocols generally enable more field devices to be connected to a particular bus, support more and faster communication between the field devices and the controller and/or allow field devices to send more and different types of information, such as information pertaining to the status and configuration of the field device itself, to the process controller. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network
Also, there is now a move within the process control industry to decentralize process control and, thereby, simplify process controllers. Decentralized control is obtained by having field mounted process control devices, such as valve positioners, transmitters, etc. perform one or more process control functions using what are typically referred to as function blocks and by then communicating data across a bus structure for use by other process control devices (or function blocks) in performing other control functions. To implement these control functions, each process control device typically includes a microprocessor having the capability to implement one or more function blocks as well as the ability to communicate with other process control devices using a standard and open communication protocol. In this manner, field devices can be interconnected within a process control network to communicate with one another and to perform one or more process control functions forming a control loop without the intervention of a centralized process controller. The all-digital, two-wire bus protocol now being promulgated by the Fieldbus Foundation, known as the FOUNDATION™ Fieldbus (hereinafter “Fieldbus”) protocol is one open communication protocol that allows devices made by different manufacturers to interoperate and communicate with one another via a standard bus to effect decentralized control within a process.
Because digital communication protocols and decentralized control schemes (such as that used in the Fieldbus control environment) are so new, processes which implement these protocols typically do so only to a limited extent. As a result, newer process controllers, such as the DeltaV™ process controller manufactured by Fisher-Rosemont Systems support both analog and digital communication protocols and hardware and can be programmed to implement control in a process that includes field devices which communicate using standard analog protocols, such as the 4-20 mA protocol, and one or more of the newer digital protocols, such as the Fieldbus protocol.
However, problems have arisen when trying to integrate control of analog and digital field devices, and particularly Fieldbus field devices, in a process control network using a centralized controller. Because the control functions for analog field devices and some digital field devices are implemented within the centralized process controller, all the necessary information about these field devices is received by and stored within the centralized process controller. This in turn enables the centralized process controller to integrate control of the different analog and digital field devices, to display the current configuration and state of the portions of the process control network in which these devices are located, to make changes to the process control network configuration with respect to these devices, etc. However, when a decentralized control scheme, such as the Fieldbus control scheme, is used in a part of the process, the centralized process controller no longer needs or has direct access to all of the current information being used by or associated with the process control devices subject to the decentralized control. In fact, some decentralized process control protocols, such as the Fieldbus protocol, are specifically designed so that it is not necessary to send information to a centralized process controller on a regular basis. This fact makes it difficult for the centralized process controller to integrate the control of the analog or other digital field devices with the field devices subject to decentralized control. Also, it makes it difficult for the centralized process controller to model or display the current state of the devices subject to decentralized control or within a decentralized portion of the process control network.
In fact, for the centralized process controller to receive information from the decentralized control portion of the process control network, the field devices (or function blocks) within that portion of the process must be specifically configured to send information directly to the centralized process controller (which means that the centralized process controller must receive and track all of this information, most of which is not necessary for operation of the centralized process controller). Alternatively, the centralized process controller must actively request to receive the information it needs. Because such a request is not given a high priority in, for example, the Fieldbus protocol, by the time the centralized process controller receives the requested information, this information may be out of date. Furthermore, it is difficult, if not impossible for the centralized process controller to request and receive data that is active or current at a specified time. Instead, the centralized process controller only receives the data active at the time the request reaches the field device. Still further, communication between the centralized process controller and field devices within the decentralized portion of the process is highly specialized and requires considerable knowledge of the decentralized control protocol, which makes it difficult for the designer of the centralized process control routine to implement this communication on an as needed basis.